How do i select multiple object Using Python Script?
If you want to select all objects at once, then use: bpy.ops.object.select_all(action='SELECT') (select_all operator). In case you want to only select a subset of objects you have to iterate through the list of scene objects (bpy.context.scene.objects) or through the list of objects in the blend-file:

Blender 2.7x

# Deselect all objects
bpy.ops.object.select_all(action='DESELECT')

for o in bpy.data.objects:
    # Check for given object names
    if o.name in ("Cube.026","Cube.027","Cube.028"):
        o.select = True


Blender 2.8x

# Deselect all objects
bpy.ops.object.select_all(action='DESELECT')

for o in bpy.data.objects:
    # Check for given object names
    if o.name in ("Cube.026","Cube.027","Cube.028"):
        o.select_set(True)